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Improving the flotation performance of an oxidized bituminous coal source
Minerals Engineering 142 (2019) 105937
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Improving the flotation performance of an oxidized bituminous coal source Raghav Dube, Rick Honaker
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T
Department of Mining Engineering, University of Kentucky, Lexington, KY, 40508
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxidation Flotation Collector Optimization Dispersion
Numerous studies have been conducted describing the challenges with oxidized coal flotation. Common industrial collectors such as fuel oil are typically ineffective under natural conditions for the flotation of oxidized coal sources such as the Coalburg seam from the Central Appalachia coalfields of the U.S. Low plant flotation recovery values of less than 30% were realized in part due to the weak surface hydrophobicity and the clay coating that results from a reduced isoelectric point of the coal and low slurry pH values. Elevated pH values and the use of a model collector containing carboxylic groups improved flotation recovery values by nearly 45 absolute percentage points. Operating at pH values around 7.5 to disperse the clay slimes and provide more surface active dimer complexes produced from a carboxyl collector resulted in excellent flotation recovery values for a hard-to-float coal. Due to the frothing properties of the carboxyl collector, column flotation with froth washing was needed to provide product ash values as low as 7.5%.
1. Introduction Previous studies describe the coal oxidation process based on two categories: gaseous or dry oxidation and chemical or wet oxidation (Somasundaran et al., 1991). When coal is exposed to air, moisture is lost and weathering takes place. The loss of moisture reduces the protective water layer on the coal surface thereby allowing the surface to be exposed to atmospheric oxygen which results in significant quality deterioration. Oxidation further induces fissuring which results in swelling of the coal particles (Berkowitz and Klein, 1989). Oxidation of the coal particles also affects the surface charge directly by decreasing the point of zero charge (PZC) and increasing the magnitude of the negative charge over a wider range of slurry pH values. For most materials including coal, reducing the slurry pH to lower values by adding hydronium ions eventually results in a positive surface charge while adding hydroxyl ions creates a negatively charge surface at higher pH values (Angle and Hamza, 1989; Hunter, 1981; Laskowski, 1987; Shaw, 1992). The pH value corresponding to a transition from a positively charged surface to a negatively charged surface is known as the PZC. For coal particles, the pH value corresponding to the PZC depends on multiple factors including coal rank, degree of oxidation, oxygen functional groups and ash mineral content (Fuerstenau et al., 1988; Wei et al., 2017). In general, bituminous and anthracite coals have PZC values within the pH range of 4.5–6.0 and oxidation reduces the PZC to lower values (Wen and Sun, 1977). For oxidized coals, the lower degree of surface hydrophobicity and
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the propensity of the coal surface to attract slimes under natural pH conditions result in low flotation recovery performances. Kaolinite is the most dominant type of clay contained in fine coal (Vassilev and Vassileva, 1996). The surface charge of kaolinite is complex with the basal plane (face) of the particle maintaining a constant negative charge as a function of pH while the edge surface has a positive charge below pH 7.2 and negative above the same pH value (Olphan, 1977). As a result, clay particles form a slime coating on the surface of the coal particles at pH values below 7.2 due to interactions between negatively charged coal surfaces with positively charged clay edges or between positively charged coal surfaces and negatively charged basal planes of the clay particles (Arnold and Aplan, 1986; Honaker et al., 2005). The slime coating suppresses surface hydrophobicity and thus floatability. A range of chemicals are used in coal flotation to enhance floatability while also improving selectivity. Fuel oil no. 2 is the most commonly used collector while aliphatic alcohols such as MIBC (methyl isobutyl carbinol) and glycols are commonly used as frothers. Industrial practice as well as research findings have shown that straight chain aliphatic oily collectors like fuel oil no. 2 perform efficiently for unoxidized coals and poorly for oxidized coals. Ionic collectors are preferred in the case of oxidized coals. Wen and Sun (1981) found that a composite mixture of fuel oil no. 2 and fuel oil no. 6 at a ratio of 4:1 was effective for floating oxidized coal. Cyclic hydrocarbons such as cyclodecane (C10H18) have also been found to provide slightly better performance as compared to dodecane (Harris et al., 1995). It was discovered that the introduction of a benzene ring to the collector can
Corresponding author. E-mail address: [email protected] (R. Honaker).
https://doi.org/10.1016/j.mineng.2019.105937 Received 14 April 2019; Received in revised form 12 August 2019; Accepted 13 August 2019 Available online 20 August 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
Minerals Engineering 142 (2019) 105937
R. Dube and R. Honaker
increase the collector performance as it tends to form strong π bonding with the coal aromatic structure. Also, the presence of hydrophilic groups such as ethoxy and phenol groups on the collector can further enhance the collector capability to interact with hydrophilic groups on the coal surfaces through hydrogen bonding. From the same study, ethoxylated nonyl phenol was found to be a better collector than dodecane for Illinois No. 6 coal (Harris et al., 1995) as it could be adsorbed both on hydrophobic and hydrophilic surfaces (Aston et al., 1989). A similar study showed that a particular series of esters could provide a substantial improvement in flotation performance with oxidized coals (Jia et al., 2002). Oxidized coal was treated with a wide range of tetrahydrofurfuryl esters (THF), which contained a hydrocarbon chain as well as oxygenated functional groups. The results showed that THF-17en (containing oleate) and THF-11 (containing laurate) provided significant improvement in flotation recovery of oxidized coals. Water soluble polymers containing polypropylene oxides (PPO) and polyethylene oxides (PEO) were also found to be beneficial for the flotation of oxidized and low rank coals (Chander et al., 1994; Polat, 1995; Polat and Chander, 1998; Polat et al., 2003). These polymers not only acted as surface modifiers by adsorption onto coal surfaces but also served as an emulsifier, which assisted in the dispersion of oil within the slurry. Collectors with oxygen functional groups, such as α-furanacrylic, can form hydrogen bonding with oxidized coal surfaces, thereby enhancing their flotation characteristics (Gui et al., 2017; Tian et al., 2019). Recent studies have shown that adding hydrophobic particles such as magnetic particles (Honaker et al., 2017) or candle soot (Xia et al., 2018) can improve coal froth stability and flotation kinetics. In other studies, vegetable oil, which is mainly comprised of a wide variety of fatty acids, was found to be an effective collector for coal flotation. Olive and soybean oil provided a substantial increase in combustible recovery when floating low rank coals (Alonso et al., 2000). The oilbased collectors were effective for agglomerating coal fines at very low oil concentrations through interactions between the fatty acids and coal surfaces by hydrogen bonding (Alonso, et al., 2002; Denby et al., 2002; Valdés and Garcia, 2006). Fatty acids are considered to be weak electrolyte collectors as indicated by a dissociation constant (pKa) in the range of 4.2–5.2 (Fuerstenau, 1982). Therefore, these acids dissociate mainly in various species at basic medium (pH > 7.5) (Kulkarni and Somasundaran, 1980). Oleic acid also tends to form an acid soap dimer complex, which is highly surface active with maximum concentration of these acid soap dimers occurring at around pH 7.8 (Ananthpadmanabhan et al., 1979). The presence of the dimer complexes minimizes the surface tension of the solution, which further enhances the flotation process. Recent research has shown that mixing two collectors could benefit the flotation of lower rank coals (Miao et al., 2018; Xia et al., 2016). This paper describes the use of a model collector comprised of a mixture of oleic acid and fuel oil as collector to float oxidized bituminous coal. Species of oleic acid present at specific slurry pH values help in recovering the oxidized coal particles while producing a clean coal concentrate with a relatively low ash content.
Table 1 Particle size-by-size weight and quality distribution on a dry basis for the Coalburg flotation feed. Particle Size Fraction (µm)
Weight (%)
Ash (%)
T.Sulfur (%)
Heat Value (btu/lb)
+ 300 −300 + 150 −150 + 75 −75 + 45 −45
0.39 2.30 9.00 2.67 85.65
7.64 4.65 11.71 20.18 60.56
0.79 0.70 0.73 0.59 0.28
14,248 14,345 13,995 12,352 4565
Total
100.00
53.60
0.35
5884
Peerless seam coal, excellent coal recovery and grade was obtained thereby confirming the fact that the reason for the poor flotation performance when treating Coalburg seam coal was directly associated with the coal itself and not the process. The Coalburg coal slurry sample contained 6.5% solids by weight and 53.6% ash. Nearly 86% of the flotation feed was finer than 45 µm and contained 60.56% ash suggesting the presence of a significant amount of ultrafine coal particles in the flotation feed (Table 1). An ultimate analysis performed on the flotation feed sample showed 14.4% oxygen presence in the flotation feed samples. 2.2. Coal surface characterization To provide a preliminary assessment of the degree of surface oxidation, the oxidation number was measured using an indirect technique. The oxidation extent of the flotation feed was evaluated using the ASTM D5263-93 method in which a single laser beam having a wavelength of 520 nm was passed through an alkali extract solution. The oxidation number for the Coalburg flotation sample was 28 which indicated a significant amount of surface oxidation. A value for coal sources with excellent floatability characteristics is typically around 95. The degree of hydrophobicity was directly assessed by measuring the contact angle using the sessile drop technique and selected raw feed coal lumps which were cut and polished using two-micron aluminum oxide polishing powder. Multiple measurements were obtained which resulted in a mean of 47.3° with a standard deviation of 5.4°. The limited flotation performance observed in the plant and in laboratory tests do not agree with expectations of a coal with a contact angle of 47.3°, which is likely due to the naturally low slurry pH value of around 5.0 caused by the release of humic acids and the resulting surface coverage of clay slimes. Oxidation also impacts the surface potential of the particles as indicated by a study of the zeta potential as a function of pH. From the study, a point-of-zero charge (PZC) below pH 2.0 was discovered for the Coalburg flotation feed coal. The coal surfaces were found to have a negative charge throughout the pH range. In general, the PZC values for good floating coal sources occur around a pH range of 5–7, as shown in Fig. 1 for the un-oxidized Peerless seam coal. The plant slurry pH was 5.6 which provided a highly negative surface charge for the Coalburg seam coal while the Peerless seam coal had a near neutral surface charge. This finding provided an explanation for the very weak flotation response for the Coalburg coal as slime coatings would be more pronounced on the surface of the Coalburg coal compared to the easilyfloatable Peerless seam coal.
2. Experimental 2.1. Sample acquisition and analysis A coal sample was collected from the flotation feed stream of an operating coal preparation plant treating bituminous coal located in central Appalachian coal field in U.S. The source of the flotation feed was the overflow stream of a bank of 381-mm diameter raw coal classifiers. The coal origin was the Coalburg seam, which is well known to have a high degree of oxidation and thus difficult-to-float characteristics. The flotation circuit at the plant was used to treat metallurgical coal from the Peerless coal seam while the slurry containing Coalburg seam coal was diverted to the thickener. When treating
2.3. Flotation rate tests To estimate the effect of different chemicals on particle flotation rate, a series of flotation rate tests were performed (ASTM D5114-90) using a four-liter laboratory Denver flotation cell. In these tests, slurry was first conditioned with collector for 14 min and subsequently with frother for 1 min. After conditioning, the air valve was adjusted to provide an aeration rate of 3 lpm. The product samples were collected 2
Minerals Engineering 142 (2019) 105937
R. Dube and R. Honaker
Zeta Potential (mV)
60
used to maintain the desired froth height and bias flow rate.
40
Coalburg Seam Coal
20
Peerless Seam Coal
3. Results and discussion 3.1. Preliminary reagent evaluation
0 Flotation rate tests performed with fuel oil as collector and MIBC as frother confirmed the poor flotation performance realized in the plant operation. As shown in Fig. 3(a), commercial alcohol type frother F110 provided a slight improvement although recovery remained at a relatively low level of 35% after 8 min of flotation time. The relationship between recovery and product ash shown in Fig. 3(b) indicated that both recovery and selectivity was improved by increasing the frother dosage to 20 ppm which is somewhat higher than industrial practice. Since conventional reagents failed to provide significant improvements, a series of rate tests were performed with several commercially available reagents. As shown in Fig. 4, a carboxylic-type frother provided a significant increase in combustible recovery when using fuel oil as the collector at a dosage of 0.45 kg/t dosage. Another series of flotation tests were conducted to compare the performance of commercial collectors that were known to have a mixture of various carboxylic compounds. The collectors provided a further improvement in flotation rate and thus recovery. Using a collector identified as FECO7 and MIBC as frother, the 4-minute flotation recovery was 70% which was twice the value achieved using fuel oil/MIBC and five absolute percentage points greater than the value achieved using the fuel oil/carboxy frother combination. This combination also provided superior selectivity to both the standard fuel oil/alcohol frother combination and other commercial collectors as shown in Fig. 4. Based on the positive results obtained using carboxylic collectors, oleic acid was selected as a model collector for further investigation. Oleic acid is a popular ingredient for many commercial collectors and frothers.
-20 -40 -60 0
3
6
9
12
15
Slurry pH Fig. 1. A comparison of the zeta potential values of a Coalburg seam coal with poor flotation response and a Peerless seam coal with excellent flotation characteristics.
in 30 s time intervals over a period of 8 min. The product samples were filtered, dried, weighed and analyzed for their respective ash contents according to the ASTM D5142 standard. 2.4. Column flotation tests A laboratory flotation column unit measuring 240 cm in height and 5 cm in diameter was used in the investigation. The resulting length-todiameter ratio of 40:1 provided near plug flow conditions which is an optimum environment for bubble-particle collisions. Bubbles were generated by pumping a portion of the tailings slurry through a static mixer. Air was injected into the recirculating line before the static mixer at a rate that provided a gas velocity of 1.6 cm/sec in the column. Prior to the test, the slurry was conditioned with collector in the feed sump for 15 min. Frother was injected into the feed slurry just before entrance into the column cell at a rate that provided an overall concentration in the cell of 20 ppm. Volumetric feed rate was controlled using a peristaltic pump as shown in Fig. 2. Wash water was added in the froth zone to remove entrainment using a flowmeter to obtain a flow rate of 400 ml/min. A controller, pressure transducer and a control valve were
3.2. Model collector studies Past results have shown that the performance of fatty acids have a strong correlation with slurry pH (Ananthpadmanabhan et al., 1979; Kulkarni and Somasundaran, 1980). Activation of the dimer complex
Fig. 2. Laboratory column flotation setup. 3
Minerals Engineering 142 (2019) 105937
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100
90
FO 0.45 kg/t MIBC 10 ppm
80
FO 0.45 kg/t MIBC 20 ppm
70
FO 0.45 kg/t F110 20 ppm
(a)
60 50 40 30 20
FO 0.45 kg/t MIBC 20 ppm
80
FO 0.45 kg/t F110 20 ppm
70 60
(b)
50 40 30 20 10
10 0
FO 0.45 kg/t MIBC 10 ppm
90
Combustible Recovery (%)
Combustible Recovery (%)
100
0 0
2
4
6
Time (minutes)
8
0
10
60
40
20
Product Ash (%)
Fig. 3. Results from kinetic rate tests using fuel oil as the collector.
100
FECO3 0.45 kg/t F110 20 ppm FECO7 0.45 kg/t F110 20 ppm GP 0.45 kg/t A644 20 ppm FO 0.45 kg/t A644 20 ppm FO 0.45 kg/t F110 20 ppm
90 80 70
90
Combustible Recovery (%)
Combustible Recovery (%)
100
60 50 40 30 20
80 70 60 50 40 30 20 10
10
0
0 0
2
4
Time (minutes)
6
0
8
1
2
3 4 5 Time (minutes)
6
7
8
OLA + FO (1:1) 0.45 kg/t MIBC 20 ppm pH 7.5 OLA 0.68 kg/t MIBC 20 ppm pH 7.5 FO 0.68 Kg/t MIBC 20 ppm pH 7.5 OLA + FO (1:1) 0.45 kg/t MIBC 20 ppm pH 5.6 OLA 0.45 Kg/t MIBC 20 ppm pH 5.6 FO 0.45 kg/t MIBC 20 ppm pH 5.6
Fig. 4. Comparison of kinetic rate tests with some commercial collectors and frothers under natural pH conditions.
associated with oleic acid has been proven to be optimum around a slurry pH value of 7.8. Therefore, a series of experiments were performed using oleic acid as a collector over a range of slurry pH values. Laboratory grade 99% pure oleic acid was obtained from Fisher Scientific for the experimental work. An industry grade MIBC frother with known chemical composition was chosen for these experiments and used at a concentration of 20 ppm. The collector dosage was maintained at 0.45 kg/t. As shown in Fig. 5, oleic acid did not provide acceptable flotation performance when used alone with MIBC. The flotation rate and recovery values were similar to those obtained with fuel oil at the natural slurry pH of 5.6. However, at the same pH value, a 1:1 ratio of fuel oil and oleic acid at a combined dosage of 0.45 kg/t provided a significant improvement in recovery which was likely due to better dispersion of the oleic acid. When the slurry pH was increased to 7.5, almost 71% combustible recovery was obtained after 4 min of flotation (Fig. 5). Increasing the slurry pH value to 7.5 elevated the recovery rate significantly for both the fuel oil and oleic acid systems. Recovery was generally improved by 30 absolute percentage points after four minutes of flotation. As previously mentioned, a slurry pH value around 7.5 is ideal for producing the dimer complexes which are surface active and beneficial to improving surface hydrophobicity. In addition, contributing factors to the improved recovery are associated with the dispersion of the clay particles and removal of the humic acid from the coal particles (Jena, et al., 2008; Lowenhaupt and Gray, 1980). The pH effect was examined further for a 1:1 ratio of oleic acid-fuel oil mixture by varying the pH between 5.8 and 9.0. As shown in Fig. 6, maximum recovery was achieved between pH values of 7.5 and 8.0. Recovery dropped under pH conditions above 8.0 and below 7.5 as the
Combustible Recovery (%)
Fig. 5. Comparison of kinetic rate tests performed using oleic acid and fuel oil mixture as collector at different slurry pH values (OLA represents oleic acid, FO represents Standard fuel oil no. 2).
100 90 80 70 60 50 40 30 20 10 0
1 min flotation 2 min flotation 4 min flotation 8 min flotation
5
6
7
8
Slurry pH
9
10
Fig. 6. The effect of slurry pH on coal recovery using a 1:1 blend of fuel oil and oleic acid at a combined dosage of 0.45 kg/t and MIBC at a concentration of 20 ppm.
activity of the dimer complexes decrease sharply beyond slurry pH 8 (Kulkarni and Somasundaran, 1980). For a more detailed investigation of the interaction effects of the collector dosage, slurry pH and collector composition, a parametric experimental test program was conducted using a three-level Box4
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80
Table 2 Parameters and the corresponding range in values evaluated as part of a test program that used a three-level experimental design. Variable Name
Units
Low Level
High Level
A B C
Slurry pH Fuel oil % in collector Collector dosage
– % kg/t
6.0 25 0.45
9.0 75 0.9
Combustible Recovery (%)
Variable
70
Behnken design test and parameter ranges shown in Table 2. A total of 17 experiments were conducted in randomized order to reduce experimental bias. Combustible recovery values achieved after four minutes of flotation time were used to assess the parametric effects since the values reflected the impact on the majority of the floatable material (Figs. 3–5). The test results were used to develop an empirical model that described the parameter and parameter interaction effects on combustible recovery. The following cubic model for the four-min flotation recovery (R4) was found to be statistically valid with an overall P-value of less than 0.0001 and adjusted coefficient of determination (R2) value equal to 0.995:
60 50 40 30
4 min Comb. Rec
20
2 min Comb. Rec. 1 min Comb. Rec.
10 0 0
20
40 60 % Fuel Oil in Collector
80
100
Fig. 8. Combustible recovery as a function of fuel oil content in the collector mixture; total collector dosage of 0.68 kg/t (1.5 lb/t), MIBC dosage of 20 ppm, slurry pH of 7.5.
set of experiments were fixed at 0.68 kg/t and 20 ppm, respectively. From the test results presented in Fig. 8, an increase in fuel oil percent in the collector mixture resulted in a steady improvement in combustible recovery up to fuel oil composition of around 80% after which combustible recovery decreased sharply. Oleic acid has a limited solubility of around 10−5 M at a pH value of around pH 7.5 (Laskowski, 1988; Laskowski and Nyamekye, 1994). The limited solubility may explain the reason for optimum collector composition with just 20% oleic acid. Also, the weak solubility of oleic acid at lower pH values may explain the reason for the poor performance when oleic acid alone was used at pH 5.6. The parametric test results clearly showed that almost 74% combustible recovery could be achieved when using optimum conditions. However, elevated product ash values were another issue that required attention prior to being accepted for industrial practical applications. The four-minute product ash content varied from 14% to 23% with the higher values obtained under optimum recovery conditions. A likely explanation involves an increase in water recovery to the froth product which caused a corresponding elevation in the amount of hydraulically entrained clay particles reporting to the flotation concentrate. To achieve lower product ash values, a series of experiments were conducted using a laboratory-scale flotation column.
R 4 = 108.6 + 2.5B - 980.7 A - 0.72 AB + 240.7 AC - 1.21 A2 + 18.4 C2 + 0.05 A2B - 14.4 A2C Fig. 7 shows that flotation recovery has a strong correlation with slurry pH with maximum recovery values observed at a slurry pH value of 7.5 irrespective of collector dosage and collector composition. It was also observed that the four-minute flotation recovery improved with an increase in fuel oil content in fuel oil/oleic acid collector mixture. The four-minute recovery values were found to increase slightly when either fuel oil content in the collector mixture was increased from 25% to 75% or collector dosage was increased from 0.45 kg/t to 0.90 kg/t. Therefore, it could be concluded that, between slurry pH values of 7.5 and 7.8, an optimum combustible recovery value of around 72% is achievable with a collector mixture containing 75% fuel oil (Fig. 7). This finding further supports the previous observations in this publication and elsewhere that the acid soap dimer complex is more surface active around a slurry pH value of 7.5. An optimum ratio of oleic acid and fuel was not achieved in the parametric tests which may be due to the fact that the fuel oil percent in the collector mixture was only varied from 25% to 75%. Therefore, to identify an optimum fuel oil content in the collector, a series of kinetic rate tests were conducted at a slurry pH value of 7.5 with varying fuel oil ratios from 0% to 100%. Dosages of collector and frother used in this
Fig. 7. (a) Response surface for 4 min combustible recovery as a function of fuel oil% in collector and slurry pH at 0.68 kg/t (1.5 lb/t) collector dosage (b) Response surface for 4 min combustible recovery as function of collector dosage and slurry pH at 50% fuel oil in collector mixture. 5
Minerals Engineering 142 (2019) 105937
90
100
80
90
Combustible Recovery (%)
Combustible Recovery (%)
R. Dube and R. Honaker
70 60 50 40 30 20 10 0
0
5
10
15
20
25
30
80 70 60 50 40 30
Column without pH adjustment OLA + FO (1:4) 0.68 kg/t Column pH 7.5 OLA + FO (1:4) 0.68 kg/t Column Test pH 7.5 GP767 + FO (1:4) 0.68 kg/t Column Test pH 7.5 FC9700 + FO (1:4) 0.68 kg/t
20 10
35
0
Product Ash (%)
0
Conventional cell pH 7.5 OLA + FO (1:4) 0.68 kg/t
500
1000
1500
2000
Volumetric Feed Rate (ml/min)
Column without pH adjustment OLA + FO (1:4) 0.68 kg/t Column pH 7.5 OLA + FO (1:4) 0.68 kg/t
Fig. 10. Combustible recovery obtained from the column flotation cell as a function of volumetric feed rate when treating Coalburg seam coal over a range of test conditions and reagent types.
Column Test pH 7.5 GP767 + FO (1:4) 0.68 kg/t Column Test pH 7.5 FC9700 + FO (1:4) 0.68 kg/t
Fig. 9. Comparison of column flotation and conventional cell separation performances under optimum conditions.
difficulty observed in an operating preparation plant. Under natural pH conditions of around 5.6, the Coalburg seam coal had a weak response to flotation efforts with recovery values below 15% and product ash values significantly higher than 10%. Test results proved that the presence of oxygen groups such as carboxyl groups within flotation reagents can enhance the floatability of oxidized coal particles. A model collector system of oleic acid and fuel oil mixture was evaluated. A comprehensive study was performed to quantify the effect and optimize the values of the oleic acid-fuel oil blend, collector dosage and slurry pH. The optimized settings were utilized in a series of laboratory flotation column experiments to evaluate the full potential of maximizing recovery while achieving the lowest possible product ash content. The adjustment of slurry pH from its natural value of 5.6–7.5 was found to provide the largest impact on combustible recovery. In conventional cell rate tests, the recovery achieved after 4 min of flotation was increased from 28% to 60% by the pH adjustment. This impact can be explained by a release of humic acids from the coal surface and dispersion of clay particles in slightly basic medium. However, raising the pH to 9 resulted in a significant decrease in recovery, which was most likely due to the elevated negative surface charges resulting in higher electrostatic repulsive energies between particles and bubbles. A model system of oleic acid and fuel oil was studied in detail which showed that a standard fuel oil-to-oleic acid ratio of 4:1 maximized flotation performance for an oxidized coal at a slurry pH of 7.5. Using the blend in conventional cell tests resulted in a recovery of nearly 74% of the combustibles which equated to an increase of over 50 absolute percentage points from the initial tests and 15 points after adjusting the slurry pH. Fatty acids similar to oleic acid could be used as an effective collector for oxidized coals. However, product ash values were elevated due to increased water recovery and hydraulic entrainment. Column flotation was found to be effective at achieving the elevated recovery values while producing low ash content products due to its ability to eliminate or minimize hydraulic entrainment. With the use of a laboratory column flotation cell, a product containing around 7.5% ash was achieved while recovering 75% of the combustible material from an oxidized coal source using an optimum slurry pH value of 7.5 and a 4:1 ratio of fuel oil and oleic acid.
3.3. Column flotation evaluation Column flotation tests were performed using optimum conditions derived from conventional cell test results, i.e., (1) slurry pH of 7.5 and (2) fuel oil-to-oleic acid ratio of 4:1. The overall collector dosage was maintained at 0.68 kg/t while the frother addition rate provided a concentration in the column of 20 ppm. A product ash of less than 8% was achieved using the flotation column with nearly 77% recovery of the combustible material as shown in Fig. 9. The performance represented a significant improvement given the high product ash values of over 10% and recovery values of around 15% that were typically realized when treating the coal in the operating preparation plant. The low product ash contents were realized due to the ability to eliminate the hydraulically entrained clay through dispersion at a pH of 7.5 and froth washing in the column flotation cell while enhancing flotation recovery using the 4:1 ratio blend of fuel oil and oleic acid. Two commercially available fatty acid type collectors were also tested in the flotation column while maintaining the pH value at 7.5 and the collector mixture at the 1:4 ratio with fuel oil. Recovery values and the corresponding product ash contents were found to have a similar trend as those obtained with fuel oil and the oleic acid mixture. As shown in Fig. 9, collector FC9700 provided slightly improved selectivity with a maximum combustible recovery of 78% at a product ash content of around 9%. The advantage of the commercial collectors with fuel oil were the higher volumetric feed rates at given combustible recovery values compared to the performances obtained using the oleic acid and fuel oil mixture (Fig. 10). These results indicate that the commercial fatty acids enhanced flotation rates of the oxidized coal thereby increasing the throughput capacity of the flotation cells. Given that the commercial collectors are blends of various fatty acids, a wide variety of acid dimers corresponding to each type of fatty acid are present in the slurry at pH 7.5 which could increase the overall activity of the collector. Furthermore, the blend of fatty acids has excellent frother properties which results in the formation of a finer bubble size distributions and thus improved flotation rates.
Acknowledgement
4. Conclusions
The authors gratefully acknowledge SNF Flomin for the financial support which made this work possible. Special appreciation is extended to Alpha Natural Resources for arranging plant visits and collecting samples for the study.
Oxidized coal particles exhibit poor flotation characteristics with standard flotation chemicals such as fuel oil as collector and MIBC as frother. Flotation tests performed on oxidized coal extracted from the Coalburg seam in the U.S. Central Appalachia coalfields confirmed the 6
Minerals Engineering 142 (2019) 105937
R. Dube and R. Honaker
References
Laskowski, J., 1987. Coal electrokinetics: The origin of charge at coal/water interface. Div. Fuel Chem. American Chemical Society, United States 32(CONF-870410-). Laskowski, J., 1988. Weak-electrolyte type collectors. Proceedings Copper-87 International Conference on Mineral Processing and Process Control. Laskowski, J., Nyamekye, G., 1994. Colloid chemistry of weak electrolyte collectors: the effect of conditioning on flotation with fatty acids. Int. J. Miner. Proc. 40 (3), 245–256. Lowenhaupt, D., Gray, R., 1980. The alkali-extraction test as a reliable method of detecting oxidized metallurgical coal. Int. J. Coal Geology. 1 (1), 63–73. Miao, Z., Xing, Y., Gui, X., Cao, Y., Wang, T., 2018. Anthracite coal flotation using dodecane and nonyl benzene. Int. J. Coal Prep. Util. 38 (8), 393–401. Olphan, H.V., 1977. An Introduction to Clay Colloid Chemistry. John Wiley & Sons, New York. Polat, H., 1995. The Use of PEO/PPO Tri Block Co-polymers to Enhance Fine Coal Cleaning by Flotation (Thesis). Pennsylvania State University. Polat, H., Chander, S., 1998. Interaction between physical and chemical variables in the flotation of low rank coals. Miner. Metal. Proc. 15 (1), 41–47. Polat, M., Polat, H., Chander, S., 2003. Physical and chemical interactions in coal flotation. Int. J. Miner. Proc. 72 (1), 199–213. Shaw, D.J., 1992. Introduction to Surface and Colloid Chemistry. ButterworthHeinemann, London, pp. 1–150. Somasundaran, P., Roberts, C., Ramesh, R., 1991. Effects of oxidizing methods on the flotation of coal. Miner. Eng. 4 (1), 43–48. Tian, Q., Zhang, Y., Li, G., Wang, Y., 2019. Application of carboxylic acid in low-rank coal flotation. Int. J. Coal Prep. Util. 39 (1), 44–53. Valdés, A.F., Garcia, A.B., 2006. On the utilization of waste vegetable oils (WVO) as agglomerants to recover coal from coal fines cleaning wastes (CFCW). Fuel 85 (5), 607–614. Vassilev, S.V., Vassileva, C.G., 1996. Occurrence, abundance and origin of minerals in coals and coal ashes. Fuel Process. Technol. 48 (2), 85–106. Wei, W., Kumar, A., Holuszko, M. E., Mastalerz, M.D., 2017. Selection of reagents based on surface chemistry as derived from micro-FTIR mapping of coal surface to facilitate selectivity in coal flotation. Wen, W., Sun, S., 1977. Electrokinetic study on the amine flotation of oxidized coal. Trans. Soc. Min. Eng 262(2). Wen, W., Sun, S., 1981. An electrokinetic study on the oil flotation of oxidized coal. Sep. Sci. Technol. 16 (10), 1491–1521. Xia, W., Li, Y., Nguyen, A.V., 2018. Improving coal flotation using the mixture of candle soot and hydrocarbon oil as a novel flotation collector. J. Cleaner Prod. 195, 1183–1189. https://doi.org/10.1016/j.jclepro.2018.06.020. Xia, W., Ni, C., Xie, G., 2016. Effective flotation of lignite using a mixture of dodecane and 4-dodecylphenol (ddp) as a collector. Int. J. Coal Prep. Util. 36 (5), 262–271. https:// doi.org/10.1080/19392699.2015.1113956.
Alonso, M., Castaño, C., Garcia, A., 2000. Performance of vegetable oils as flotation collectors for the recovery of coal from coal fines wastes. Coal Prep. 21 (4), 411–420. Alonso, M., Valdés, A., Martı́nez-Tarazona, R., Garcia, A., 2002. Coal recovery from fines cleaning wastes by agglomeration with colza oil: a contribution to the environment and energy preservation. Fuel Process. Technol. 75 (2), 85–95. Arnold, B.J., Aplan, F., 1986. The effect of clay slimes on coal flotation: Part I. The nature of the clay. Int. J. Miner. Process. 17, 225–242. Ananthpadmanabhan, K., Somasundaran, P., Healy, T., 1979. Chemistry of oleate and amine solutions in relation to flotation. Trans. AIME, Littleton 266, 2003–2009. Angle, C., Hamza, H., 1989. An electrokinetic study of a natural coal associated mixture of kaolinite and montmorillonite in electrolytes. Appl. Clay Sci. 4 (3), 263–278. Aston, J., Lane, J., Healy, T., 1989. The solution and interfacial chemistry of nonionic surfactants used in coal flotation. Miner. Process. Extract. Metall. Rev. 5 (1–4), 229–256. Berkowitz, N., Klein, R., 1989. Sample Selection, Aging and Reactivity of Coal. Wiley, New York. Chander, S., Polat, H., Mohal, B., 1994. Flotation and wettability of a low-rank coal in the presence of surfactants. Miner. Metal. Proc. 11 55–55. Denby, B., Elverson, C., Hal, S., 2002. The use of short chain volatile fatty acids in fine coal preparation. Fuel 81 (5), 595–603. Fuerstenau, D., Rosenbaum, J., You, Y., 1988. Electrokinetic behavior of coal. Energy Fuels 2 (3), 241–245. Fuerstenau, M., 1982. Chemistry of collectors in solution. In: S. African Inst. Mining and Metal. Principles of Flotation, pp. 1–16. Gui, X., Xing, Y., Wang, T., Cao, Y., Miao, Z., Xu, M., 2017. Intensification mechanism of oxidized coal flotation by using oxygen-containing collector α-furanacrylic acid. Powder Technol. 305, 109–116. https://doi.org/10.1016/j.powtec.2016.09.058. Harris, G., Diao, J., Fuerstenau, D., 1995. Coal flotation with nonionic surfactants. Coal Prep. 16 (3–4), 135–147. Honaker, R.Q., Luttrell, G.H., Yoon, R.-H., 2005. Ultrafine coal cleaning using selective hydrophobic coagulation. Coal Prep.: Int. J. 25 (2), 81–98. Honaker, R., Saracoglu, M., Huang, Q., 2017. Evaluation of a novel coal flotation improvement approach with the addition of hydrophobic magnetic particles. Int. J. Coal Prep. Util. 1–17. https://doi.org/10.1080/19392699.2017.1419208. Hunter, R.J., 1981. Zeta Potential in Colloid Science: Principles and Applications. Academic Press, London. Jena, M., Biswal, S., Rudramuniyappa, M., 2008. Study on flotation characteristics of oxidised Indian high ash sub-bituminous coal. Int. J. Miner. Proc. 87 (1), 42–50. Jia, R., Harris, G.H., Fuerstenau, D.W., 2002. Chemical reagents for enhanced coal flotation. Coal Prep. 22 (3), 123–149. Kulkarni, R.D., Somasundaran, P., 1980. Flotation chemistry of hematite/oleate system. Colloids Surf. 1 (3–4), 387–405. https://doi.org/10.1016/0166-6622(80)80025-4.
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Minerals Engineering journal homepage: www.elsevier.com/locate/mineng
Improving the flotation performance of an oxidized bituminous coal source Raghav Dube, Rick Honaker
⁎
T
Department of Mining Engineering, University of Kentucky, Lexington, KY, 40508
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxidation Flotation Collector Optimization Dispersion
Numerous studies have been conducted describing the challenges with oxidized coal flotation. Common industrial collectors such as fuel oil are typically ineffective under natural conditions for the flotation of oxidized coal sources such as the Coalburg seam from the Central Appalachia coalfields of the U.S. Low plant flotation recovery values of less than 30% were realized in part due to the weak surface hydrophobicity and the clay coating that results from a reduced isoelectric point of the coal and low slurry pH values. Elevated pH values and the use of a model collector containing carboxylic groups improved flotation recovery values by nearly 45 absolute percentage points. Operating at pH values around 7.5 to disperse the clay slimes and provide more surface active dimer complexes produced from a carboxyl collector resulted in excellent flotation recovery values for a hard-to-float coal. Due to the frothing properties of the carboxyl collector, column flotation with froth washing was needed to provide product ash values as low as 7.5%.
1. Introduction Previous studies describe the coal oxidation process based on two categories: gaseous or dry oxidation and chemical or wet oxidation (Somasundaran et al., 1991). When coal is exposed to air, moisture is lost and weathering takes place. The loss of moisture reduces the protective water layer on the coal surface thereby allowing the surface to be exposed to atmospheric oxygen which results in significant quality deterioration. Oxidation further induces fissuring which results in swelling of the coal particles (Berkowitz and Klein, 1989). Oxidation of the coal particles also affects the surface charge directly by decreasing the point of zero charge (PZC) and increasing the magnitude of the negative charge over a wider range of slurry pH values. For most materials including coal, reducing the slurry pH to lower values by adding hydronium ions eventually results in a positive surface charge while adding hydroxyl ions creates a negatively charge surface at higher pH values (Angle and Hamza, 1989; Hunter, 1981; Laskowski, 1987; Shaw, 1992). The pH value corresponding to a transition from a positively charged surface to a negatively charged surface is known as the PZC. For coal particles, the pH value corresponding to the PZC depends on multiple factors including coal rank, degree of oxidation, oxygen functional groups and ash mineral content (Fuerstenau et al., 1988; Wei et al., 2017). In general, bituminous and anthracite coals have PZC values within the pH range of 4.5–6.0 and oxidation reduces the PZC to lower values (Wen and Sun, 1977). For oxidized coals, the lower degree of surface hydrophobicity and
⁎
the propensity of the coal surface to attract slimes under natural pH conditions result in low flotation recovery performances. Kaolinite is the most dominant type of clay contained in fine coal (Vassilev and Vassileva, 1996). The surface charge of kaolinite is complex with the basal plane (face) of the particle maintaining a constant negative charge as a function of pH while the edge surface has a positive charge below pH 7.2 and negative above the same pH value (Olphan, 1977). As a result, clay particles form a slime coating on the surface of the coal particles at pH values below 7.2 due to interactions between negatively charged coal surfaces with positively charged clay edges or between positively charged coal surfaces and negatively charged basal planes of the clay particles (Arnold and Aplan, 1986; Honaker et al., 2005). The slime coating suppresses surface hydrophobicity and thus floatability. A range of chemicals are used in coal flotation to enhance floatability while also improving selectivity. Fuel oil no. 2 is the most commonly used collector while aliphatic alcohols such as MIBC (methyl isobutyl carbinol) and glycols are commonly used as frothers. Industrial practice as well as research findings have shown that straight chain aliphatic oily collectors like fuel oil no. 2 perform efficiently for unoxidized coals and poorly for oxidized coals. Ionic collectors are preferred in the case of oxidized coals. Wen and Sun (1981) found that a composite mixture of fuel oil no. 2 and fuel oil no. 6 at a ratio of 4:1 was effective for floating oxidized coal. Cyclic hydrocarbons such as cyclodecane (C10H18) have also been found to provide slightly better performance as compared to dodecane (Harris et al., 1995). It was discovered that the introduction of a benzene ring to the collector can
Corresponding author. E-mail address: [email protected] (R. Honaker).
https://doi.org/10.1016/j.mineng.2019.105937 Received 14 April 2019; Received in revised form 12 August 2019; Accepted 13 August 2019 Available online 20 August 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.
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increase the collector performance as it tends to form strong π bonding with the coal aromatic structure. Also, the presence of hydrophilic groups such as ethoxy and phenol groups on the collector can further enhance the collector capability to interact with hydrophilic groups on the coal surfaces through hydrogen bonding. From the same study, ethoxylated nonyl phenol was found to be a better collector than dodecane for Illinois No. 6 coal (Harris et al., 1995) as it could be adsorbed both on hydrophobic and hydrophilic surfaces (Aston et al., 1989). A similar study showed that a particular series of esters could provide a substantial improvement in flotation performance with oxidized coals (Jia et al., 2002). Oxidized coal was treated with a wide range of tetrahydrofurfuryl esters (THF), which contained a hydrocarbon chain as well as oxygenated functional groups. The results showed that THF-17en (containing oleate) and THF-11 (containing laurate) provided significant improvement in flotation recovery of oxidized coals. Water soluble polymers containing polypropylene oxides (PPO) and polyethylene oxides (PEO) were also found to be beneficial for the flotation of oxidized and low rank coals (Chander et al., 1994; Polat, 1995; Polat and Chander, 1998; Polat et al., 2003). These polymers not only acted as surface modifiers by adsorption onto coal surfaces but also served as an emulsifier, which assisted in the dispersion of oil within the slurry. Collectors with oxygen functional groups, such as α-furanacrylic, can form hydrogen bonding with oxidized coal surfaces, thereby enhancing their flotation characteristics (Gui et al., 2017; Tian et al., 2019). Recent studies have shown that adding hydrophobic particles such as magnetic particles (Honaker et al., 2017) or candle soot (Xia et al., 2018) can improve coal froth stability and flotation kinetics. In other studies, vegetable oil, which is mainly comprised of a wide variety of fatty acids, was found to be an effective collector for coal flotation. Olive and soybean oil provided a substantial increase in combustible recovery when floating low rank coals (Alonso et al., 2000). The oilbased collectors were effective for agglomerating coal fines at very low oil concentrations through interactions between the fatty acids and coal surfaces by hydrogen bonding (Alonso, et al., 2002; Denby et al., 2002; Valdés and Garcia, 2006). Fatty acids are considered to be weak electrolyte collectors as indicated by a dissociation constant (pKa) in the range of 4.2–5.2 (Fuerstenau, 1982). Therefore, these acids dissociate mainly in various species at basic medium (pH > 7.5) (Kulkarni and Somasundaran, 1980). Oleic acid also tends to form an acid soap dimer complex, which is highly surface active with maximum concentration of these acid soap dimers occurring at around pH 7.8 (Ananthpadmanabhan et al., 1979). The presence of the dimer complexes minimizes the surface tension of the solution, which further enhances the flotation process. Recent research has shown that mixing two collectors could benefit the flotation of lower rank coals (Miao et al., 2018; Xia et al., 2016). This paper describes the use of a model collector comprised of a mixture of oleic acid and fuel oil as collector to float oxidized bituminous coal. Species of oleic acid present at specific slurry pH values help in recovering the oxidized coal particles while producing a clean coal concentrate with a relatively low ash content.
Table 1 Particle size-by-size weight and quality distribution on a dry basis for the Coalburg flotation feed. Particle Size Fraction (µm)
Weight (%)
Ash (%)
T.Sulfur (%)
Heat Value (btu/lb)
+ 300 −300 + 150 −150 + 75 −75 + 45 −45
0.39 2.30 9.00 2.67 85.65
7.64 4.65 11.71 20.18 60.56
0.79 0.70 0.73 0.59 0.28
14,248 14,345 13,995 12,352 4565
Total
100.00
53.60
0.35
5884
Peerless seam coal, excellent coal recovery and grade was obtained thereby confirming the fact that the reason for the poor flotation performance when treating Coalburg seam coal was directly associated with the coal itself and not the process. The Coalburg coal slurry sample contained 6.5% solids by weight and 53.6% ash. Nearly 86% of the flotation feed was finer than 45 µm and contained 60.56% ash suggesting the presence of a significant amount of ultrafine coal particles in the flotation feed (Table 1). An ultimate analysis performed on the flotation feed sample showed 14.4% oxygen presence in the flotation feed samples. 2.2. Coal surface characterization To provide a preliminary assessment of the degree of surface oxidation, the oxidation number was measured using an indirect technique. The oxidation extent of the flotation feed was evaluated using the ASTM D5263-93 method in which a single laser beam having a wavelength of 520 nm was passed through an alkali extract solution. The oxidation number for the Coalburg flotation sample was 28 which indicated a significant amount of surface oxidation. A value for coal sources with excellent floatability characteristics is typically around 95. The degree of hydrophobicity was directly assessed by measuring the contact angle using the sessile drop technique and selected raw feed coal lumps which were cut and polished using two-micron aluminum oxide polishing powder. Multiple measurements were obtained which resulted in a mean of 47.3° with a standard deviation of 5.4°. The limited flotation performance observed in the plant and in laboratory tests do not agree with expectations of a coal with a contact angle of 47.3°, which is likely due to the naturally low slurry pH value of around 5.0 caused by the release of humic acids and the resulting surface coverage of clay slimes. Oxidation also impacts the surface potential of the particles as indicated by a study of the zeta potential as a function of pH. From the study, a point-of-zero charge (PZC) below pH 2.0 was discovered for the Coalburg flotation feed coal. The coal surfaces were found to have a negative charge throughout the pH range. In general, the PZC values for good floating coal sources occur around a pH range of 5–7, as shown in Fig. 1 for the un-oxidized Peerless seam coal. The plant slurry pH was 5.6 which provided a highly negative surface charge for the Coalburg seam coal while the Peerless seam coal had a near neutral surface charge. This finding provided an explanation for the very weak flotation response for the Coalburg coal as slime coatings would be more pronounced on the surface of the Coalburg coal compared to the easilyfloatable Peerless seam coal.
2. Experimental 2.1. Sample acquisition and analysis A coal sample was collected from the flotation feed stream of an operating coal preparation plant treating bituminous coal located in central Appalachian coal field in U.S. The source of the flotation feed was the overflow stream of a bank of 381-mm diameter raw coal classifiers. The coal origin was the Coalburg seam, which is well known to have a high degree of oxidation and thus difficult-to-float characteristics. The flotation circuit at the plant was used to treat metallurgical coal from the Peerless coal seam while the slurry containing Coalburg seam coal was diverted to the thickener. When treating
2.3. Flotation rate tests To estimate the effect of different chemicals on particle flotation rate, a series of flotation rate tests were performed (ASTM D5114-90) using a four-liter laboratory Denver flotation cell. In these tests, slurry was first conditioned with collector for 14 min and subsequently with frother for 1 min. After conditioning, the air valve was adjusted to provide an aeration rate of 3 lpm. The product samples were collected 2
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Zeta Potential (mV)
60
used to maintain the desired froth height and bias flow rate.
40
Coalburg Seam Coal
20
Peerless Seam Coal
3. Results and discussion 3.1. Preliminary reagent evaluation
0 Flotation rate tests performed with fuel oil as collector and MIBC as frother confirmed the poor flotation performance realized in the plant operation. As shown in Fig. 3(a), commercial alcohol type frother F110 provided a slight improvement although recovery remained at a relatively low level of 35% after 8 min of flotation time. The relationship between recovery and product ash shown in Fig. 3(b) indicated that both recovery and selectivity was improved by increasing the frother dosage to 20 ppm which is somewhat higher than industrial practice. Since conventional reagents failed to provide significant improvements, a series of rate tests were performed with several commercially available reagents. As shown in Fig. 4, a carboxylic-type frother provided a significant increase in combustible recovery when using fuel oil as the collector at a dosage of 0.45 kg/t dosage. Another series of flotation tests were conducted to compare the performance of commercial collectors that were known to have a mixture of various carboxylic compounds. The collectors provided a further improvement in flotation rate and thus recovery. Using a collector identified as FECO7 and MIBC as frother, the 4-minute flotation recovery was 70% which was twice the value achieved using fuel oil/MIBC and five absolute percentage points greater than the value achieved using the fuel oil/carboxy frother combination. This combination also provided superior selectivity to both the standard fuel oil/alcohol frother combination and other commercial collectors as shown in Fig. 4. Based on the positive results obtained using carboxylic collectors, oleic acid was selected as a model collector for further investigation. Oleic acid is a popular ingredient for many commercial collectors and frothers.
-20 -40 -60 0
3
6
9
12
15
Slurry pH Fig. 1. A comparison of the zeta potential values of a Coalburg seam coal with poor flotation response and a Peerless seam coal with excellent flotation characteristics.
in 30 s time intervals over a period of 8 min. The product samples were filtered, dried, weighed and analyzed for their respective ash contents according to the ASTM D5142 standard. 2.4. Column flotation tests A laboratory flotation column unit measuring 240 cm in height and 5 cm in diameter was used in the investigation. The resulting length-todiameter ratio of 40:1 provided near plug flow conditions which is an optimum environment for bubble-particle collisions. Bubbles were generated by pumping a portion of the tailings slurry through a static mixer. Air was injected into the recirculating line before the static mixer at a rate that provided a gas velocity of 1.6 cm/sec in the column. Prior to the test, the slurry was conditioned with collector in the feed sump for 15 min. Frother was injected into the feed slurry just before entrance into the column cell at a rate that provided an overall concentration in the cell of 20 ppm. Volumetric feed rate was controlled using a peristaltic pump as shown in Fig. 2. Wash water was added in the froth zone to remove entrainment using a flowmeter to obtain a flow rate of 400 ml/min. A controller, pressure transducer and a control valve were
3.2. Model collector studies Past results have shown that the performance of fatty acids have a strong correlation with slurry pH (Ananthpadmanabhan et al., 1979; Kulkarni and Somasundaran, 1980). Activation of the dimer complex
Fig. 2. Laboratory column flotation setup. 3
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100
90
FO 0.45 kg/t MIBC 10 ppm
80
FO 0.45 kg/t MIBC 20 ppm
70
FO 0.45 kg/t F110 20 ppm
(a)
60 50 40 30 20
FO 0.45 kg/t MIBC 20 ppm
80
FO 0.45 kg/t F110 20 ppm
70 60
(b)
50 40 30 20 10
10 0
FO 0.45 kg/t MIBC 10 ppm
90
Combustible Recovery (%)
Combustible Recovery (%)
100
0 0
2
4
6
Time (minutes)
8
0
10
60
40
20
Product Ash (%)
Fig. 3. Results from kinetic rate tests using fuel oil as the collector.
100
FECO3 0.45 kg/t F110 20 ppm FECO7 0.45 kg/t F110 20 ppm GP 0.45 kg/t A644 20 ppm FO 0.45 kg/t A644 20 ppm FO 0.45 kg/t F110 20 ppm
90 80 70
90
Combustible Recovery (%)
Combustible Recovery (%)
100
60 50 40 30 20
80 70 60 50 40 30 20 10
10
0
0 0
2
4
Time (minutes)
6
0
8
1
2
3 4 5 Time (minutes)
6
7
8
OLA + FO (1:1) 0.45 kg/t MIBC 20 ppm pH 7.5 OLA 0.68 kg/t MIBC 20 ppm pH 7.5 FO 0.68 Kg/t MIBC 20 ppm pH 7.5 OLA + FO (1:1) 0.45 kg/t MIBC 20 ppm pH 5.6 OLA 0.45 Kg/t MIBC 20 ppm pH 5.6 FO 0.45 kg/t MIBC 20 ppm pH 5.6
Fig. 4. Comparison of kinetic rate tests with some commercial collectors and frothers under natural pH conditions.
associated with oleic acid has been proven to be optimum around a slurry pH value of 7.8. Therefore, a series of experiments were performed using oleic acid as a collector over a range of slurry pH values. Laboratory grade 99% pure oleic acid was obtained from Fisher Scientific for the experimental work. An industry grade MIBC frother with known chemical composition was chosen for these experiments and used at a concentration of 20 ppm. The collector dosage was maintained at 0.45 kg/t. As shown in Fig. 5, oleic acid did not provide acceptable flotation performance when used alone with MIBC. The flotation rate and recovery values were similar to those obtained with fuel oil at the natural slurry pH of 5.6. However, at the same pH value, a 1:1 ratio of fuel oil and oleic acid at a combined dosage of 0.45 kg/t provided a significant improvement in recovery which was likely due to better dispersion of the oleic acid. When the slurry pH was increased to 7.5, almost 71% combustible recovery was obtained after 4 min of flotation (Fig. 5). Increasing the slurry pH value to 7.5 elevated the recovery rate significantly for both the fuel oil and oleic acid systems. Recovery was generally improved by 30 absolute percentage points after four minutes of flotation. As previously mentioned, a slurry pH value around 7.5 is ideal for producing the dimer complexes which are surface active and beneficial to improving surface hydrophobicity. In addition, contributing factors to the improved recovery are associated with the dispersion of the clay particles and removal of the humic acid from the coal particles (Jena, et al., 2008; Lowenhaupt and Gray, 1980). The pH effect was examined further for a 1:1 ratio of oleic acid-fuel oil mixture by varying the pH between 5.8 and 9.0. As shown in Fig. 6, maximum recovery was achieved between pH values of 7.5 and 8.0. Recovery dropped under pH conditions above 8.0 and below 7.5 as the
Combustible Recovery (%)
Fig. 5. Comparison of kinetic rate tests performed using oleic acid and fuel oil mixture as collector at different slurry pH values (OLA represents oleic acid, FO represents Standard fuel oil no. 2).
100 90 80 70 60 50 40 30 20 10 0
1 min flotation 2 min flotation 4 min flotation 8 min flotation
5
6
7
8
Slurry pH
9
10
Fig. 6. The effect of slurry pH on coal recovery using a 1:1 blend of fuel oil and oleic acid at a combined dosage of 0.45 kg/t and MIBC at a concentration of 20 ppm.
activity of the dimer complexes decrease sharply beyond slurry pH 8 (Kulkarni and Somasundaran, 1980). For a more detailed investigation of the interaction effects of the collector dosage, slurry pH and collector composition, a parametric experimental test program was conducted using a three-level Box4
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80
Table 2 Parameters and the corresponding range in values evaluated as part of a test program that used a three-level experimental design. Variable Name
Units
Low Level
High Level
A B C
Slurry pH Fuel oil % in collector Collector dosage
– % kg/t
6.0 25 0.45
9.0 75 0.9
Combustible Recovery (%)
Variable
70
Behnken design test and parameter ranges shown in Table 2. A total of 17 experiments were conducted in randomized order to reduce experimental bias. Combustible recovery values achieved after four minutes of flotation time were used to assess the parametric effects since the values reflected the impact on the majority of the floatable material (Figs. 3–5). The test results were used to develop an empirical model that described the parameter and parameter interaction effects on combustible recovery. The following cubic model for the four-min flotation recovery (R4) was found to be statistically valid with an overall P-value of less than 0.0001 and adjusted coefficient of determination (R2) value equal to 0.995:
60 50 40 30
4 min Comb. Rec
20
2 min Comb. Rec. 1 min Comb. Rec.
10 0 0
20
40 60 % Fuel Oil in Collector
80
100
Fig. 8. Combustible recovery as a function of fuel oil content in the collector mixture; total collector dosage of 0.68 kg/t (1.5 lb/t), MIBC dosage of 20 ppm, slurry pH of 7.5.
set of experiments were fixed at 0.68 kg/t and 20 ppm, respectively. From the test results presented in Fig. 8, an increase in fuel oil percent in the collector mixture resulted in a steady improvement in combustible recovery up to fuel oil composition of around 80% after which combustible recovery decreased sharply. Oleic acid has a limited solubility of around 10−5 M at a pH value of around pH 7.5 (Laskowski, 1988; Laskowski and Nyamekye, 1994). The limited solubility may explain the reason for optimum collector composition with just 20% oleic acid. Also, the weak solubility of oleic acid at lower pH values may explain the reason for the poor performance when oleic acid alone was used at pH 5.6. The parametric test results clearly showed that almost 74% combustible recovery could be achieved when using optimum conditions. However, elevated product ash values were another issue that required attention prior to being accepted for industrial practical applications. The four-minute product ash content varied from 14% to 23% with the higher values obtained under optimum recovery conditions. A likely explanation involves an increase in water recovery to the froth product which caused a corresponding elevation in the amount of hydraulically entrained clay particles reporting to the flotation concentrate. To achieve lower product ash values, a series of experiments were conducted using a laboratory-scale flotation column.
R 4 = 108.6 + 2.5B - 980.7 A - 0.72 AB + 240.7 AC - 1.21 A2 + 18.4 C2 + 0.05 A2B - 14.4 A2C Fig. 7 shows that flotation recovery has a strong correlation with slurry pH with maximum recovery values observed at a slurry pH value of 7.5 irrespective of collector dosage and collector composition. It was also observed that the four-minute flotation recovery improved with an increase in fuel oil content in fuel oil/oleic acid collector mixture. The four-minute recovery values were found to increase slightly when either fuel oil content in the collector mixture was increased from 25% to 75% or collector dosage was increased from 0.45 kg/t to 0.90 kg/t. Therefore, it could be concluded that, between slurry pH values of 7.5 and 7.8, an optimum combustible recovery value of around 72% is achievable with a collector mixture containing 75% fuel oil (Fig. 7). This finding further supports the previous observations in this publication and elsewhere that the acid soap dimer complex is more surface active around a slurry pH value of 7.5. An optimum ratio of oleic acid and fuel was not achieved in the parametric tests which may be due to the fact that the fuel oil percent in the collector mixture was only varied from 25% to 75%. Therefore, to identify an optimum fuel oil content in the collector, a series of kinetic rate tests were conducted at a slurry pH value of 7.5 with varying fuel oil ratios from 0% to 100%. Dosages of collector and frother used in this
Fig. 7. (a) Response surface for 4 min combustible recovery as a function of fuel oil% in collector and slurry pH at 0.68 kg/t (1.5 lb/t) collector dosage (b) Response surface for 4 min combustible recovery as function of collector dosage and slurry pH at 50% fuel oil in collector mixture. 5
Minerals Engineering 142 (2019) 105937
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100
80
90
Combustible Recovery (%)
Combustible Recovery (%)
R. Dube and R. Honaker
70 60 50 40 30 20 10 0
0
5
10
15
20
25
30
80 70 60 50 40 30
Column without pH adjustment OLA + FO (1:4) 0.68 kg/t Column pH 7.5 OLA + FO (1:4) 0.68 kg/t Column Test pH 7.5 GP767 + FO (1:4) 0.68 kg/t Column Test pH 7.5 FC9700 + FO (1:4) 0.68 kg/t
20 10
35
0
Product Ash (%)
0
Conventional cell pH 7.5 OLA + FO (1:4) 0.68 kg/t
500
1000
1500
2000
Volumetric Feed Rate (ml/min)
Column without pH adjustment OLA + FO (1:4) 0.68 kg/t Column pH 7.5 OLA + FO (1:4) 0.68 kg/t
Fig. 10. Combustible recovery obtained from the column flotation cell as a function of volumetric feed rate when treating Coalburg seam coal over a range of test conditions and reagent types.
Column Test pH 7.5 GP767 + FO (1:4) 0.68 kg/t Column Test pH 7.5 FC9700 + FO (1:4) 0.68 kg/t
Fig. 9. Comparison of column flotation and conventional cell separation performances under optimum conditions.
difficulty observed in an operating preparation plant. Under natural pH conditions of around 5.6, the Coalburg seam coal had a weak response to flotation efforts with recovery values below 15% and product ash values significantly higher than 10%. Test results proved that the presence of oxygen groups such as carboxyl groups within flotation reagents can enhance the floatability of oxidized coal particles. A model collector system of oleic acid and fuel oil mixture was evaluated. A comprehensive study was performed to quantify the effect and optimize the values of the oleic acid-fuel oil blend, collector dosage and slurry pH. The optimized settings were utilized in a series of laboratory flotation column experiments to evaluate the full potential of maximizing recovery while achieving the lowest possible product ash content. The adjustment of slurry pH from its natural value of 5.6–7.5 was found to provide the largest impact on combustible recovery. In conventional cell rate tests, the recovery achieved after 4 min of flotation was increased from 28% to 60% by the pH adjustment. This impact can be explained by a release of humic acids from the coal surface and dispersion of clay particles in slightly basic medium. However, raising the pH to 9 resulted in a significant decrease in recovery, which was most likely due to the elevated negative surface charges resulting in higher electrostatic repulsive energies between particles and bubbles. A model system of oleic acid and fuel oil was studied in detail which showed that a standard fuel oil-to-oleic acid ratio of 4:1 maximized flotation performance for an oxidized coal at a slurry pH of 7.5. Using the blend in conventional cell tests resulted in a recovery of nearly 74% of the combustibles which equated to an increase of over 50 absolute percentage points from the initial tests and 15 points after adjusting the slurry pH. Fatty acids similar to oleic acid could be used as an effective collector for oxidized coals. However, product ash values were elevated due to increased water recovery and hydraulic entrainment. Column flotation was found to be effective at achieving the elevated recovery values while producing low ash content products due to its ability to eliminate or minimize hydraulic entrainment. With the use of a laboratory column flotation cell, a product containing around 7.5% ash was achieved while recovering 75% of the combustible material from an oxidized coal source using an optimum slurry pH value of 7.5 and a 4:1 ratio of fuel oil and oleic acid.
3.3. Column flotation evaluation Column flotation tests were performed using optimum conditions derived from conventional cell test results, i.e., (1) slurry pH of 7.5 and (2) fuel oil-to-oleic acid ratio of 4:1. The overall collector dosage was maintained at 0.68 kg/t while the frother addition rate provided a concentration in the column of 20 ppm. A product ash of less than 8% was achieved using the flotation column with nearly 77% recovery of the combustible material as shown in Fig. 9. The performance represented a significant improvement given the high product ash values of over 10% and recovery values of around 15% that were typically realized when treating the coal in the operating preparation plant. The low product ash contents were realized due to the ability to eliminate the hydraulically entrained clay through dispersion at a pH of 7.5 and froth washing in the column flotation cell while enhancing flotation recovery using the 4:1 ratio blend of fuel oil and oleic acid. Two commercially available fatty acid type collectors were also tested in the flotation column while maintaining the pH value at 7.5 and the collector mixture at the 1:4 ratio with fuel oil. Recovery values and the corresponding product ash contents were found to have a similar trend as those obtained with fuel oil and the oleic acid mixture. As shown in Fig. 9, collector FC9700 provided slightly improved selectivity with a maximum combustible recovery of 78% at a product ash content of around 9%. The advantage of the commercial collectors with fuel oil were the higher volumetric feed rates at given combustible recovery values compared to the performances obtained using the oleic acid and fuel oil mixture (Fig. 10). These results indicate that the commercial fatty acids enhanced flotation rates of the oxidized coal thereby increasing the throughput capacity of the flotation cells. Given that the commercial collectors are blends of various fatty acids, a wide variety of acid dimers corresponding to each type of fatty acid are present in the slurry at pH 7.5 which could increase the overall activity of the collector. Furthermore, the blend of fatty acids has excellent frother properties which results in the formation of a finer bubble size distributions and thus improved flotation rates.
Acknowledgement
4. Conclusions
The authors gratefully acknowledge SNF Flomin for the financial support which made this work possible. Special appreciation is extended to Alpha Natural Resources for arranging plant visits and collecting samples for the study.
Oxidized coal particles exhibit poor flotation characteristics with standard flotation chemicals such as fuel oil as collector and MIBC as frother. Flotation tests performed on oxidized coal extracted from the Coalburg seam in the U.S. Central Appalachia coalfields confirmed the 6
Minerals Engineering 142 (2019) 105937
R. Dube and R. Honaker
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